1. Field of the Invention
The present invention is concerned with the improvements in and relating to a crawler-belt vehicle (half-crawler vehicle) having front wheels furnished with tires and rear wheels with crawlers.
2. Description of the Related Art
Half-crawler vehicles are known as disclosed, for example, in Japanese Patent Laid-open Publication No. SHO 60-143189, entitled "OFF-ROAD VEHICLE", and Japanese Patent Laid-open Publication No. SHO 59-164270, entitled "FOUR-WHEEL-DRIVE CRAWLER VEHICLE".
The off-road vehicle disclosed in Japanese Patent Laid-open Publication No. SHO 60-143189 includes, as shown in FIGS. 2 and 3 of the same publication, front wheels attached to the front portion of a vehicle body frame and furnished with balloon tires. Rear wheels and intermediate wheels both furnished with balloon tires are attached to the rear portion of the vehicle body frame, with rubber crawler belts extending around the rear wheels and the intermediate wheels.
The four-wheel-drive crawler vehicle disclosed in Japanese Patent Laid-open Publication No. SHO 59-164270 includes, as shown in FIG. 1 of the same publication, front wheels attached to the front portion of a vehicle body, and rear wheels and intermediate wheels both furnished with tires and attached to the rear portion of the vehicle body, with rubber crawler belts running around the rear wheels and the intermediate wheels.
According to the half-crawler vehicle disclosed in Japanese Patent Laid-open Publication No. SHO 60-143189, however, since the front wheels serving as steered wheels and the rear wheels serving as driven wheels have substantially the same diameter, the front wheels acquire a smaller tread area than the crawler belts when the vehicle is running on snow. The front wheels are, therefore, liable to sink in the snow to such an extent that a sink-in-snow portion of the front wheels has a large area when viewed from the side of the wheels. Thus, at steering the front wheels are subjected to a great resistance of snow, failing to improve the turning performance of the half crawler vehicle. This problem becomes serious particularly when the vehicle is traveling over an untrodden field covered with virgin snow.
The same may be said of the four-wheel-drive crawler vehicle disclosed in Japanese Patent Laid-open Publication No. SHO 59-164270.
Particularly for the half crawler vehicles used for traveling over untrodden fields covered with new-fallen snow, a high breakthrough performance (capability of breaking a passage through a snow-covered untrodden field, for example) is required.
In the four-wheel-drive crawler vehicle disclosed in Japanese Patent Laid-open Publication No. SHO 59-164270, the rear wheel and the corresponding intermediate wheel on each side are attached in many cases to the vehicle body via a swingable sub-beam. In these cases, an arrangement is generally employed in which the intermediate wheel is attached to the outer end of a support shaft or spindle attached to a portion of the sub-beam extending toward the forward direction of travel of the vehicle.
With the half-crawler vehicle thus arranged, the sub-beam is subjected to a bending moment due to an unbalanced load applied when the intermediate wheel is loaded. For instance, when the half-crawler vehicle is running straight on snow at high speeds, the left and right sub-beams are slightly flexed or deformed, by bending moments applied via the intermediate wheels due to resistance of the snow, in such a manner that the sub-beams are spread or flared at their front end (in a lateral outward direction of the vehicle).
As a consequence, the intermediate wheels are toeing out, in which instance each crawler belt while running tends to displace in a lateral outward direction of the vehicle body as if the belt is running between conical rollers. This phenomenon cannot be avoided without difficulties.
The crawler belt is provided with a number of teeth (side guide portions) on its inside surface. When the crawler belt is displaced laterally and outwardly, the side guide portions are forced into pressure contact with the tires and thus increase a running resistance of the crawler belt. The running resistance becomes greater as the vehicle running speed increases.
In addition, the half-crawler vehicle is constructed such that the rear wheels and the intermediate wheels cannot be steered. Accordingly, an imaginary center of turn of each crawler is located substantially centrally between the rear wheel and the intermediate wheel on the same side. The imaginary turning center is determined by a dynamic weight distribution (weight distribution while the vehicle is running) between the front wheels, rear wheels and intermediate wheels.
Accordingly, the crawlers tend to cause a side slip (a slip in the vehicle width direction) at positions directly beneath the rear wheels and the intermediate wheels. Since an undue slip would result in detachment of the crawler belts, the amount of side slip and a side slip produced thereby should preferably be minimized.